In many antenna applications, for example such as for use with military aircraft and vehicles, an antenna with a broad bandwidth is required. For such applications, the so-called "frequency-independent antenna" ("FI antenna") commonly has been employed. See for example, V. H. Rumsey, Frequency Independent Antennas, Academic Press, New York, N.Y., 1966. Such frequency-independent antennas typically have a radiating, or driven element with spiral, or log-periodic geometry that enables the FI antenna to transmit and receive signals over a wide band of frequencies, typically on the order of a 9:1 ratio or more (a bandwidth of 900%). European Patent Application No. 86301175.5 of R. H. DuHamel entitled "Dual Polarized Sinuous Antennas", published Oct. 22, 1986, publication No. 0198578, discloses frequency-independent antennas with sinuous structures.
In a frequency-independent antenna, a lossy cylindrical cavity is positioned to one side of the antenna element so that when transmitting, energy effectively is radiated outwardly from the antenna only from one side of the antenna element (the energy radiating from the other side of the antenna element being dissipated in the cavity). However, high-performance military aircraft, and other applications as well, require that the antenna be mounted substantially flush with its exterior surface, in this case the skin of the aircraft. This undesirably requires that the cavity portion of the FI antenna be mounted within the structure of the aircraft, necessitating that a substantial hole be formed therein to accommodate the cylindrical cavity, which typically is two inches deep and several inches in diameter for microwave frequencies. Also, the use of a lossy cavity to dissipate radiation causes half of the radiated power to be lost, requiring a greater power input to effect a given level of power radiated outwardly from the FI antenna.
In recent years the so-called "microstrip antenna" has been developed. See for example, U.S. Pat. No. 29,911 of Munson (a reissue of U.S. Pat. No. 3,921,177) and U.S. Pat. No. 29,296 of Krutsinoer, et al (a reissue of U.S. Pat. No. 3,810,183). In a typical microstrip antenna, a solid thin metal patch is placed adjacent to a ground plane and spaced a small distance therefrom by a dielectric spacer. Microstrip antennas have generally suffered from having a narrow useful bandwidth, typically less than 10%. At least one researcher has made preliminary investigations into using a single microstrip line wound as an Archimedean spiral (C. Wood, "Curved Microstrip Lines as Compact Wideband Circularly Polarized Antennas", published in Inst. Elec. Eng. Microwaves, Optics and Acoustics, Vol. 3, pp. 5-13, January 1979). That researcher concluded, however, that the achievement of a microstrip-type antenna with a wide bandwidth analogous to the conventional spiral (of a frequency-independent antenna) was not feasible because the radiation patterns of the contemplated low-profile antenna tend to exhibit a large axial ratio.
Accordingly, it can be seen that a need yet remains for an antenna which has the dimensional characteristics of a microstrip antenna, i.e., has a low-profile, and has a broad bandwidth similar to a frequency-independent antenna. It is to the provision of such an antenna, therefore, that the present invention is primarily directed.